Method for storage of petroleum in natural underground reservoirs



Aug. 8, 1933. F. F. HILL ETAL 1,921,358

METHOD FOR STORAGE OF PETROLEUM IN NATURAL UNDERGROUND RESERVOIRS FiledFeb. 10, 1950 A TTORNE Y.

Patented Aug- 8, 1933 UNITED STATES METHOD FOR STORAGE OF PETROLEUM INNATURAL UNDERGROUND RESERVOIRS Frank F. Hill and Albert C. Rubel, LosAngeles, Calif., assignors to UnionOil Company of California, LosAngeles, Calif., a Corporation of California Application February 10,

11 Claims.

This invention relates to the storage of crude oils or of other oils indepleted or partially depleted natural underground reservoirs.

The storage of crude oil involves the use of steel tanks or concretereservoirs. When the crude oil contains any material amount of light orvolatile oils, the only safe storage, because of fire hazards, is steeltanks with appropriate roofs to minimize evaporation losses. Theseconcrete reservoirs and steel tanks are very costly.

It is an object of this invention to eliminate the expenditure requiredfor the construction of tanks for storage of oils, and at the same timeprevent evaporation losses, and eliminate fire hazards. This isaccomplished by the use of natural subterranean reservoirs, such asdepleted or partially depleted oil pools, in such a manner as willinsure the economical recovery of the stored oil from the reservoir.

It is a further object to recover a portion of the residual oil whichalways remains in the sands of the so-called depleted naturalunderground reservolrs.

Petroleum in its natural state exists in under- 0 ground reservoirs, oroil pools, associated with natural gas, and, in the majority of cases,in contact with water either at the edge or bottom of the producingformations. These fluids are contained in the interstices or pore spacesin the rock or sand strata under a pressure. This pressure, termedreservoir pressure, is the resultant of the gas pressure, the rockpressure and the hydraulic pressure, if there is such. Previous to thetime when the first well is drilled in such a reservoir or pool, thepressure-conditions are at an equilibrium. When a well is drilled andplaced on production, a region of lower pressure results at the pointwhere the well bore taps the pool. This causes a pressure gradient fromthe areas of high pressure toward the well, so that a fluid flow intothe well takes place.

The major source of energy in the propellingof the oil from theinterstices in the reservoirto the well isthe natural gas which is, to acertain extent, dissolved in the oil underthe-existing pressures, and toa certain degree mayexist-in a gaseousstate. v

Vt'hen the gas pressure is'diminished to such a point that'it isinsufllcient to cause the migration of the oil to the well bore and itselevation up the well to the ground level, the reservoir may be said tobe depleted or at least partially depleted. It is then'usual tointroduce a pump into this well and lift the oil mechanically. As analternative gas may be employed. Although the 1930. Serial No. 427,225

gas from the formation is insufficient to lift the oil, some residualgas present in the formation usually escapes with the oil when it ispumped or lifted. After a period "the oil production by means of thepump or gas lift falls below an economlsal limit and the reservoir maybe said to be exhausted. However, there is considerable oil remaining inthe formation. While estimates of the recovery of the oil from oil sandsvary, it is safe to say that as an average the maximum recovery of oilfrom formations containing the oil is but 25% of the total oiloriginally present. Consequently approximately 75% of the original oilremain in the sands which are thus saturated with oil. This oil isalways heavier and more viscous than the oil originally present, becausethe more volatile fractions and the gas have been removed. The resultantdepleted or partially depleted formation may then be spoken of as a sandsaturated with oil substantially free from volatile fractions and gas.

We have found that, after the reservoir or pool is partially oreconomically exhausted of its petroleum and natural gas content, it maybe used as a storage reservoir for crude petroleum or fluid petroleumproducts by the simulation of the former natural conditions. Thissimulation of the former natural conditions will consist of introducingthe oil under pressure into the well or wells of the reservoir or pooleither alone, or simultaneously or alternately with substantialquantities of a gas. Many ways are available for such introduction. Thisgas may be introduced through the same well bore. The oil may beintroduced into certain wells and the. gas .into other adjacent ones,and, after a period, the operation may be reversed so that the oilenters through the bore holes which previously were used for theintroduction of gas, and vice versa. This introduction of oil and'gascauses the forma-'- tion of a mixture of oil andgas under pressure insaid reservoir, which" pressure aids in the subsequent recovery of themixture. r

The partially depletedunderground reservoirs are preferably those whichare naturallysealed against the loss of "both oil and gas pressure. Ifan unsealed natural reservoirshould be selected, the oil and gasintroduced under pressure into such a reservoir would migrate away andbe lost. The selection of a reservoir to be used for-the storage shouldbe based either on tests to determine whether saidreservoir'maintainsthe fluid pressure, i. e. whether or not saidpressure is dis sipated into the surrounding strata, or on geologicaldata of said reservoir and of the surrounding strata to determinewhether said reservoir is surrounded by formations which will preventthe dissipation of the oil and/or gas introduced for storage. As amatter of economy, it is of course desirable to have legal control ofthe entire reservoir to prevent migration of the gas and oil introducedto parts of the reservoir controlled by others.

Thus, the invention may be stated as residing in the feature of storingliquids, crude oil or of other oils in wholly or partly depleted naturalsubterranean oil reservoirs or pools, preferably sealed against lossesof both oil and gas, this storage of oil being made either alone or inconjunction with the introduction and storage of natural or other gas insubstantial quantities for the purpose of aiding the subsequent recoveryof the stored oil. The introduction of oil into the oil strata is mostconveniently effected through existing oil wells, and where theintroduction occurs together with substantial quantities of gas, it maybe commingled therewith at the surface or at the bottom of the well, orin the oil stratum itself.

This invention further resides in the introduction of oil into adepleted or partially depleted formation. The oil is advantageously of alower specific gravity than that of the residual oil remaining in thepartially depleted oil pool, for the purpose of increasing the fluidityof the residual oil in the partially depleted strata, so that a greaterportion of the oil. can be ultimately recovered.

Referring to the drawing which discloses certain embodiments of theinvention by way of illustration, and which will aid the understandingof the invention:

Fig. 1 is a diagrammatic vertical section of a well, showing thepenetrated natural underground reservoir and showing the connections atthe surface of the ground to the sources of oil and gas to be introducedand stored in said natural underground reservoir; and

Fig. 2 is a diagrammatic vertical section of a natural undergroundreservoir or pool, showing three wells which have been drilled into saidreservoir, and which may be variously used for the introduction andsubsequent recovery of oil or oil and gas to be stored.

.Fig. 1 shows a means for the storage of oil and of gas through a singlewell. As shown in this figure, the casing 1 penetrates the partiallydepleted natural reservoir 2 in which the storage of the oil and gas isto be made. This reservoir is preferably properly sealed. Usually, sucha reservoir will have an impervious covering, such as for instance shaleor cap rock covering 3 to prevent the dissipation of the existing and/orintroduced oils andgas into the surrounding formations 4. The lowerportion of the casing 1, located in the stratum 2, is perforated as at5. The upper end of the casing 1 is provided with a closure head 6,through which a tubing pipe 7 passes into the casing 1, said tubing 7reaching to a point in the neighborhood of the perforations 5. Two pipes8 and 9 are connected to the upper part of said casing, said pipescarrying valves 10 and 11 respectively. The branch pipes 12 and 13 areconnected to a cross fitting 14 provided on the tubing 7. A valve 15 ispositioned in tubing 7 above said cross fitting. Valves 16 and 17 areplaced in the branch pipes 12 and 13 respectively. Pipe 8 is connectedby means of a pipe 18 to a source of gas, such as a gas line 19. Acompressor 20 is placed in the pipe 8. Pipe 9 is connected to thedischarge end of a pump 21, the suction end of which is connected bymeans of a pipe 22 to a source of the oil to be stored in the reservoir2. This may be an oil tank 23 or an oil-conveying line.

Fig. 2 shows one form of a partially depleted natural reservoir or oilpool 30 also preferably sealed to prevent the dissipation of theresidual and/or stored oil and gas into the surrounding formations 31. Aplurality of wells are assumed to be drilled into said naturalsubterranean reservoir 30; three such wells 32, 33 and 34 beingrepresented. Well 34 is shown as drilled into the high point of thereservoir, while wells 32 and 33 are shown as drilled further down theslope of the inclined formation.

In the case where only one well is used for the introduction of the oiland gas into a depleted natural reservoir, such as indicated in Fig. 1,the oil and the gas may be mixed at the surface and be introduced intothe formation in a mixed state, or they may be mixed at the bottom ofthe well, or again they may be introduced alternately into the same wellso that the oil and the gas to be stored will comingle while passingthrough the sands of the natural reservoir.

If the gas and oil to be stored are to be mixed at the surface, valves10 and 11 are closed and valves 16 and 17 are opened. The oil is forcedunder pressure from tank 23 and pipe 22 through pipes 9 and 13, by meansof the pump 21, into the cross fitting 14. At the same time, thecompressor 20 forces the gas under the same pressure, from the gassource 19, through pipes 18, 8 and 12, into the same cross fitting 14,where the oil and gas are mixed, a part of the gas being dissolved inthe oil under the existing pressure. The oil and gas thus comingled thenpass through the tubing 7 and are forced through the perforations 5 intothe natural subterranean reservoir 2 to be stored therein.

If the oil and gas are to be mixed at the bottom of the well, either ina mixing device attached to the lower end of the tubing 7 or whilepassing through the perforations 5 of the casing 1, the oil then may bepumped into the well through the tubing 7 while the compressed gas isbeing forced in through the casing. Valves 11 and 16 are then closed,while valves 10 and 17 are opened, it being understood that in all casesvalve 15 is closed during introduction of oil and/or gas. The oil isthen pumped by the pump 21 through the pipe 22, 9 and 13 and tubing 7into the bottom of the well, where it is mixed with the compressed gasforced by the compressor 20, through the pipes 18 and 8 and thecasing 1. The thus comingled oil and gas are then forced through theperforations 5 into the natural subterranean reservoir 2 by means of thepressure exerted by the pump 21 and compressor 20. It is to be observedthat this method employs the hydrostatic head of the-oil column in 7 toassist the action of pump 21 and that gas in introduced into theformation under a very high pressure, i. e. that of the head of oil in 7plus the pump pressure. This assists in the dissolving of gas in theoil.

If it is desired to force the oil and gas alternately into the well, theoil is first forced by the pump either through the casing or through thetubing, or through both the casing and the tubing by opening or closingthe respective valves 11 and 17. After a predetermined period of timethe pump is stopped, valves 11 and 17 are closed, and gas is injectedunder the necessary pressure from the gas source 19 by means of thecompressor 20. This introduction may also be made either through thecasing or through the tubing, or again through both, by the manipulationof the corresponding valves. The mixing of the oil and of the gas willoccur while the two are travelling through the interstices of thenatural underground reservoir 2.

In the case as indicated in Fig. 2, where a plurality of wells drilledinto the same natural underground reservoir are to be used for theintroduction of oil and of substantial quantities of gas for the purposeof storage, the oil and gas may be introduced into the naturalsubterranean reservoir 30 through one or more wells 34. Additionalquantities of gas may be introduced into the same strata through thewells 32 and 33 at the lower portions of the reservoir. This will causea better mixing of the gas "and oil in the sand since the gas, having asmaller specific gravity will tend to rise to the top of the dome, whilethe oil and the mixture of oil and gas will tend to occupy the lowerportions thereof. It is suite obvious that the oil alone may be pumpedinto the reservoir 30 through the well 34 and compressed gas introducedthrough the wells 32 and 33, since the tendency of the two fluids tomove due to the difference in specific gravities will cause an intimatemixing of the gas and oil. I

It is also within the scope of the invention to introduce oil or amixture of gas and oil, through the wells 32 and 33, while gas insubstantial quantities is introduced through well 34 so that a gaspocket is produced above the oil and in the upper portion of thereservoir. This pocket of gas above the oil in the strata willeventually aid in the recovery of the oil which has been introducedtherein or which has remained in the partially depleted reservoirprevious to the storage of additional quantities of gas and oil, sincethe conditions which will be found in such a structure will be similarto the conditions found in structures where the gas underpressure isfound in a pocket above the oil and thus drives the latter to the pointof lowest pressure, i. e. to the well or wells on production from saidoil structure such as wells 32 and 33.

It is obvious that different field conditions and the different objectsdesired to be obtained will cause the use of oneor the other of theabove enumerated methods, or other methods, of introducing oil and gasfor storage into the partially depleted subterranean reservoirs as willbe understood by those skilled in the art. Thus, for example, if it isdesired to obtain a good solution of the gas in the oil, the two will beintroduced into the sand in a mixed condition or, if a plurality ofwells are used, the oil will be forced into the upper portion of thereservoir while the gas will be injected under pressure into the lowerportion thereof so that the two will be thoroughly intermixed whilepassing therethrough due to the difference in specific gravities and dueto the pressure exerted on them at the wells. On the other hand ifstorage is the sole purpose, the oil may be introduced alone to be laterrecovered from the wells by gas-lift, gas drive and/or pumping. In thiscase, it is preferable to use only a few wells located adjacent to eachother to that the oil will be found localized and uncontaminate'd, thusfacilitating the ultimate recovery. It is also obvious that the type offormation v of the natural underground reservoir will limit,

at least as to rate, the total amount of storage capacity and ofultimate recovery of the stored oil and gas.

It is obvious that the pressures required for the introduction of oiland gas into the underground reservoirs will vary with the type andunderground configuration of said reservoirs and with the degree orstage to which this storage has progressed. Loose or porous formationswill require a smaller pressure for the introduction of oil and gas,while tight or compact formations will require much higher pressures. Atthe start of the storage operations, the back-pressure is low while atlater stages of said operations, when a suflicient amount of oil and gashas been introduced under pressure into the formation, a considerableback-pressure is developed requiring much higher pressure for oil andgas introduction. a It is thus obvious that with the progress of thestorage operation, greater and greater pressures will be required.

It was found that it is often advisable to introduce an oil of a lowerspecific gravity than the one naturally contained in the naturalunderground reservoir being used for storage. The low specific gravity,i. e. the light oil, has a lower viscosity and travels easier throughthe interstices of the reservoir. The light oil introduced into thevformation will also dilute the residual heavier oil remaining in thepartially depleted natural underground reservoir, thus lowering thespecific gravity of the latter and increasing the ultimate totalrecovery of oil from said formation. The introduced oil and gas, bothdue to their lower viscosities and their large amount, materially reducethe viscosity of the originally contained oil. In the passage of theintroduced oil and gas through the formation, an intimate mixture andsolution of the original oil and the introduced oil and gas occurs. Thisresults in a mixture of reduced viscosity and lower specific gravity,both aiding in the recovery of the oil and gas. Beneficial results maybe obtained by heating the oil or gas, or both of them, prior to forcingthem under pressure into the formation. This is advisable if the oil inthe formation is of such high specific gravity and of such asphaltic orparafiin content as to close the pores., This will assist in thedilution of the oil and the introduction and subsequent recovery of oiland gas since the hot oil and gas will act to aid in the mixing of theintroduced oil and of the original oil and will assist in the clearingof the closed pores. It is preferable to avoid storing gasoline andsimilar very light liquid hydrocarbons in formations containing asphaltbase oils. These light oils tend to precipitate asphalts thus cloggingthe pores of the sand. An added advantage of this method of storing gasand oil resides in the fact that, while dry gas isintroduced into theformation, the so-called wet gas, 1. e. one containing light hydrocarbonfractions, is obtained therefrom during the subsequent recovery of theoil and of the gas.

It was also found that the storage of oil with substantial quantities ofgas aids the ultimate recovery of the oil since the injected gas reducesthe amount of energy required later for the removal of said oil from thereservoir used for storage. However, it was also noticed that anextraordinarily low gas-oil ratio, i. e. the volume of gas expended forthe raising of a unit volume of oil, is obtained. This is due to thefact that an ideal, and not accidental, reservoir is created,

which has an intimately mixed solution of gas and oil. Of course, theprecautions which should be taken with any well placed on production,such as the maintenance of a low gas-oil ratio and the starting ofrecirculation of gas for gas-lift as soon as heading begins, should alsobe taken in the case of wells placed on production from such replenishednatural underground storage reservoirs.

Since the natural reservoir prior to replenishment was saturated withoil, none of the introduced oil is lost due to wetting of the sands. Itis possible not only to recover the oil introduced into the formationbut also obtain some of the residual oil due to the re-energization ofthe reservoir by the introduced oil and gas.

It is to be understood that the specific disclosures herein made aremerely illustrative of the generic invention and are not to beconsidered as in any way limiting, since many variations may be made bythose skilled in the art within the scope of the invention which weclaim.

We claim:

1. A method of storing oil and gas in and the recovery of oil and gasfrom a partially depleted natural subterranean oil reservoir having aplurality of wells drilled thereinto, which comprises introducing oiland natural gas into said reservoir for the purpose of storing the sameand. without removal of oil and gas, causing the oil and gas toaccumulate in said reservoir under pressure in simulation of an originaloil field, and subsequently recovering oil from the subterraneanreservoir.

2. A method of recovering oil from a partially depleted naturalunderground reservoir containing oil, which comprises introducing gasand a normally liquid oil of a specific gravity lighter than that of theresidual oil remaining unrecovered in the reservoir, causing the oil andgas to accumulate therein under pressure, allowing the introduced oil tomingle with the residual oil for a considerable period of time, andsubsequently recovering the mixed oils and gas.

3. A method of recovering oil from a partially depleted naturalunderground reservoir containing residual oil without suificient gas toremove said residual oil, which comprises introducing under pressure agas and a crude oil of a specific gravity lighter than that of theresidual oil remaining unrecovered in the reservoir, said pressure beingsufficient to cause the passage of the introduced oil and gas throughthe pores of the reservoir, accumulating the introduced oil and gasunder pressure in the reservoir in simulation of an original petroleumbearing zone, storing the introduced oil and gas in the reservoir for aprolonged period of time to cause commingling of the introduced oil withthe residual oil, and subsequently recovering the mixed oils.

4. A method of oil storage comprising introducing large quantities ofpetroleum oil into natural underground reservoirs, accumulating the oilin said reservoirs, and storing the oil therein in liquid form.

5. A method for storing liquids comprising introducing a liquid which isliquid under normal conditions and a gas soluble in said liquid into anatural underground reservoir and accumulating and storing the liquidtherein in liquid form.

6. A method comprising storing oil by introducing crude petroleum and agas soluble therein into a natural underground reservoir, andaccumulating said gas and oil in said reservoir under pressure withoutwithdrawal of gas and oil whereby the oil is stored in said reservoir asa liquid in contact with said gas.

7. A method comprising introducing a normally liquid petroleum andnatural gas under pressure into a natural underground reservoir withoutremoval of said gas and oil, and accumulating and storing the gas andoil in said reservoir under pressure for a prolonged period of time.

8. A method for storing oils comprising the step of introducing crudepetroleum into a depleted natural underground reservoir, accumulatingthe oil in said reservoir, allowing the oil to remain therein for aconsiderable period of time, and subsequently recovering the oil.

9. A method of storing and recovering petroleum comprising introducingpetroleum and natural gas into a natural underground reservoir,accumulating the petroleum and gas therein in commingled relation underpressure in simulation of a natural petroleum field, and subsequentlyrecovering the petroleum and gas from the reservoir.

10. A method of conserving, storing and recovering petroleum comprisingrecovering crude petroleum from one natural underground reservoir,recovering natural gas from natural sources, introducing said petroleumand gas into a depleted natural underground petroleum reservoir which isnaturally sealed against loss of oil and gas, commingling said oil andgas in said reservoir, and storing such introduced oil in the reservoirfor a prolonged period of time.

11. A method for conserving, storing and recovering petroleum comprisingrecovering crude petroleum from one natural underground reservoir inwhich the petroleum is subjected to loss by drainage, recovering naturalgas from natural sources, introducing said petroleum and gas into adepleted natural underground petroleum reservoir which is naturallysealed against loss of oil and gas, commingling said oil and gas in saidreservoir, and storing such introduced oil in the reservoir for aprolonged period of time to promote commingling of the introduced oilwith the residual oil.

FRANK F. HILL. ALBERT C. RUBEL.

